Hydraulic System with Bi-Rotational Pump

ABSTRACT

An hydraulic system employing bi-rotational pumps for directing fluid under pressure to an actuator. In one embodiment the return fluid from one port of the actuator is filtered by means of a pilot operated check valve in a by-pass circuit leading to a filter and fluid reservoir, with an additional check valve being provided to preclude flow from the by-pass circuit to the associated port of the pump. In another embodiment return flow from the head end of the actuator is directed to a filter and reservoir by a two way control valve which also functions to direct flow from the pump to pressurize the head end of the actuator. A pilot operated check valve is provided to direct flow from the head end of the actuator directly to the reservoir even should the two way control valve be inoperable. In another embodiment duplex bi-rotational pump systems provide high volume flow rate at a relatively low pressure, or a low volume flow rate at a relatively high pressure. In one embodiment of the duplex bi-rotational pump system a circuit is provided to filter the fluid returning from the actuator to the reservoir by means of a two-way flow control valve.

[ 1 Feb. 11, 1975 HYDRAULIC SYSTEM WITH Bl-ROTATIONAL PUMP [75]Inventors: Robert K. Gellatly, Los Gatos; Joel B. Meredith, San Carlos;Richard A. Green, Mountain View, all of Calif.

[73] Assignee: General Cable Corporation, San

Carlos, Calif.

[22] Filed: Feb. 14, 1974 1211 Appl. No.: 442,483

Primary Examiner--Edgar W. Geoghegan Attorney, Agent, or Firm-Flehr,Hohbach, Test,

Albritton llerbert [57] ABSTRACT An hydraulic system employingbi-rotational pumps for directing fluid under pressure to an actuator.In one embodiment the return fluid from one port of the actuator isfiltered by means of a pilot operated check valve in a by-pass circuitleading to a filter and fluid reservoir, with an additional check valvebeing provided to preclude flow from the by-pass circuit to theassociated port of the pump. In another embodiment return flow from thehead end of the actuator is directed to a filter and reservoir by a twoway control valve which also functions to direct flow from the pump topressurize the head end of the actuator. A pilot operated check valve isprovided to direct flow from the head end of the actuator directly tothe reservoir even should the two way control valve be inoperable. Inanother embodiment duplex bi-rotational pump systems provide high volumeflow rate at a relatively low pressure, or a low volume flow rate at arelatively high pressure. in one embodiment of the duplex bi-rotationalpump system a circuit is provided to filter the fluid returning from theactuator to the reservoir by means of a two-way flow control valve.

9 Claims, 6 Drawing Figures PATENIEDFEM 1 1915 v ties-4.911 SHEET 1 [IF3 NE WE uw PATENIEB FEB 1' 1 I975 SHEET 2 BF 3 PATENTED H81 1 I975 SHEET3 0F 3 mmw HYDRAULIC SYSTEM WITH'BI-ROTATIONAL PUMP BACKGROUND OF THEINVENTION This invention relates to hydraulic systems for deliveringpressurized fluid to hydraulic actuators, and in particular relates tosystems of the type described which employ bi-rotational pumps.

Various circuit designs have heretofore been developed for controllinghydraulic actuators, such as rotary or extensible hydraulic actuators orrams. Bi-rotational pumps have been developed for use in such hydrauliccircuits to reduce design complexity and cost. Conventionalbi-rotational pumps typically are of the intermeshing gear type whichare reversibly actuated with the ports of the pump alternating betweensuction and pressure functions so that an extensible actuator, forexample, can be either extended or retracted by driving the pump ineither direction.

Heretofore it has been difficult to adequately filter the hydraulicfluid in systems employing bi-rotational pumps because the return fluidfrom the actuator is returned to the suction face of the pump, with theresult that only the relatively small volume of fluid which is pumpedfrom the reservoir is filtered. In the case of a linear cylinder typeactuator, only the differential rod volume can bedumped to tank througha low micron pressure-line filter. Suction line filters must ofnecessity be limited to approximately 100 mesh screen to prevent pumpcavitation due to inadequate intake flow.

ln many cases it has been found desirable to provide a dual stagepumping system in which one stage provides a high volume, low pressureflow to the actuator while another stage provides a low volume, highpressure flow. Hydraulic systems of the type used for elevating a boom,for example, advantageously employ such a dual stage system where in onestage the boom is lowered at a fast rate under a light load, and inanother stage the boom is raised under a heavy load. Conventional dualstage systems employ unloading valves to achieve this purpose, butheretofore a dual stage system of the type described has not beenproposed which makes it feasible to employ the advantages of abirotational pumping system.

OBJECTS AND SUMMARY OF THE INVENTION It is a general object of theinvention to provide a new and improved hydraulic system employingbi-rotational pumps for supplying pressurized fluid to operate hydraulicactuators.

Another object is to provide a hydraulic system employing bi-rotationalpumps in which there is provision for filtering of the flow of fluidreturning from one port of the actuator to the reservoir.

Another object is to provide a bi-rotational pump system of the typedescribed in which the return flow of fluid from one port of theactuator is blocked from the pump by a check valve and is directed to afilter through a valve in a by-pass circuit.

Another object is to provide a bi-rotational pump system of the typedescribed which directs return flow from one port of the actuatorthrough a two way control valve to a filter and reservoir.

Another object is to provide a bi-rotational pump system of the typedescribed employing return line filtering by means ofa two way controlvalve in which provision is made for diverting the return fluid directlyto the reservoir should the control valve become inoperable for anyreason.-

Another object is to provide a duplex bi-rotational pump system of thetype described which affords a first operating stage delivering lowvolume, high pressure flow to the actuator, and another stage deliveringhigh volume, low pressure flow to the actuator.

Another object is to provide a duplex bi-rotational pump system of thetype described in which provision is made for the filtering of thereturn flow from one end of the actuator through a two way flow controlvalve.

Another object is to provide a duplex bi-rotational pump system of thetype described employing a two way flow control valve for return linefiltering in which provision is made to divert the return fluid directlyto the reservoir should the control valve become inoperable for anyreason.

The invention provides a hydraulic system employing bi-rotational pumpsfor operating hydraulic actuators. The ports of the pump are fluidlyconnected with the actuator by circuit means which includes a fluidreservoir, a suction circuit directing fluid from the reservoir totheports, a first supply circuit directing fluid from one of the pump.ports to the actuator and a second supply circuit directing fluid fromthe other pump port to the actuator. In one embodiment return fluid fromone port of the actuator is blocked from the pump by a check valve andis diverted to a filter and the fluid reservoir through a by-passcircuit under control of a pilot operated valve operating responsive tothe pressure in the first supply circuit. In another embodiment thereturn flow from one end of the actuator is diverted through the secondcircuit to the filter and reservoir by means of a two-way control valve.Should the two-way control valve become inoperable for any reason theactuator can be operated in emergency situations through the provisionofa pilot operated valve which diverts the return flow in the secondcircuit directly to the reservoir. In another embodiment the hydraulicsystem employs duplex, i.e., dual stage, bi-rotational pumps with meansto reversibly co-actuate the pumps for directing pressurized fluidthrough either of first and second supply circuits connected withseparate ports of the actuator. For high volume, low pressure operation,flow from the two pumps is combined and directed to the actuator. Forlow volume, high pressure operation, an unloading valve diverts flowfrom a port of the first stage pump to the reservoir while a duplexingcheck valve precludes flow from the second stage pump to the reservoir.During high volume, low pressure operation means is provided to open theduplexing check valve responsive to pressure in the first supplycircuit. In another embodiment of the duplex bi-rotational pump systemreturn flow from one port of the actuator is blocked from the suctionports of the pumps and directed through a filter to the reservoir bymeans of a two-way flow control valve. A pilot operated check valve isprovided to divert return fluid directly to the reservoir to permitoperation of the actuator should the flow control valve becomeinoperable for any reason.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram of oneembodiment of the invention illustrating a bi-rotational pump systemhaving provision for return line filtration by means of a check valveand by-pass circuit;

FIG. 2 is a schematic diagram of another embodiment providing abi-rotational pump system with provision for return line filtration bymeans of a check valve and by-pass circuit;

FIG. 3 is a schematic diagram of another embodiment providing abi-rotational pump system with provision for return line filtration bymeans of a two-way control valve;

FIG. 4 is a schematic diagram of another embodiment providing abi-rotational pump system with provision for return line filtration bymeans of a two-way control valve;

FIG. 5 is a schematic diagram of another embodiment providing a duplexbi-rotational pump system; and

FIG. 6 is a schematic diagram of another embodiment providing a duplexbi-rotational pump system with provision for return line filtration bymeans of a two-way control valve.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the drawings FIG. 1illustrates generally at 10 an hydraulic system adapted for operating asuitable hydraulic actuator 11 such as a rotary actuator or extensiblecylinder or ram of the type used, for example, in the machine toolindustry.

Hydraulic system 10 includes a suitable bi-rotational pump 12,preferably of a design which incorporates intermeshing gears rotatablymounted within a pump housing. The pump is reversibly actuated by meansof a suitable motor 13, such as an electric motor, coupled with the pumpthrough a drive shaft 14. Rotation of drive shaft 14 by the motor in onerotational direction or first operation mode draws fluid under suctionfrom a first pump port 16 and directs the fluid under pressure throughthe other port 17. Rotation of the drive shaft in the oppositerotational sense or second mode reverses the fluid flow and draws fluidunder suction from the second port 17 and directs it under pressurethrough the first port 16.

Circuit means is provided for fluidly interconnecting the pump portswith the actuator. The circuit means includes a fluid reservoir 18connected with the pump ports through a bifurcated suction circuit 19.The fluid is drawn in this circuit through a suitable coarse filter 21,preferably of a size on the order of one hundred mesh, and is directedthrough a first branch 22 having a check valve 23 into the first port 16during the first mode of operation, and through a branch conduit 24through a check valve 25 into the second port 17 during the second modeof operation. The respective check valves 23, 25 function to precludereturn flow into the reservoir through whichever branch is connectedwith the pressure face of the pump, depending upon the mode ofoperation.

The circuit means further includes a first supply circuit comprisingconduits 27, 28 which interconnect the first port of pump 12 with afirst port 29 ofthe actuator. A second supply circuit is provided andincludes conduits 31, 32, 33 for interconnecting the second port 17 ofthe pump with the second port 34 of the actuator. One-way valve meanscomprising a check valve 36 is provided in conduit 31 of the secondsupply circuit to direct flow therethrough only in a direction from thepump to the actuator.

A bifurcated return circuit is provided in the circuit means andcomprises first and second branch conduits 37, 38 connected withrespective first and second supply circuits and also connected with acommon conduit 39 leading to a fluid reservoir 41 through a suitablefine filter 42, preferably of a mesh size on the order of 40 microns.While separate reservoirs 18, 41 are illustrated schematically, theinvention contemplates that the reservoirs could be interconnected, or asingle reservoir provided, for the suction and pressure relief circuits.Suitable pressure relief valves 45, 43 are provided in respective firstand second branch conduits of the return circuit. Preferably the reliefvalves comprise normally closed valves which sense pressure inrespective first and second supply circuits and open when the pressuresin the respective circuits exceed a predetermined value that is selectedto preclude overpressure damage to the system.

The circuit means further includes a by-pass circuit comprising aconduit 44 and pilot operated check valve 46 for directing fluid fromconduit 32 of the second supply circuit to the return circuit and filter42 during the second mode of pump operation in which the first supplycircuit is pressurized. One end of conduit 44 is connected in thecircuit on the outlet side of check valve 36, and the other end isconnected with the return circuit on the outlet side of pressure reliefvalve 43. A pilot conduit 47 connects pilot operated check valve 46 withconduit 28 of the first supply circuit for communicating the pressure ofthe first supply circuit to valve 46 for opening the same after suchpressure exceeds a predetermined value.

The use and operation of the hydraulic system of FIG. 1 will bedescribed assuming, for example, that actuator is an extensiblehydraulic ram having a rod end and a head end. The port 29 is at the rodend of the ram and the port 34 is at the head end. A valve having asetting to open at 800 psi is provided as the first pressure reliefvalve 45, and a valve having a setting to open at 1,500 psi is providedas the second pressure relief valve 43. For extending the ram motor 13is energized to drive pump 12 in its first operational mode drawingfluid under suction through first port 16 and directing it underpressure through second port 17. Fluid is drawn into the first port fromreservoir 18 and coarse filter 21 through the branch of suction circuit22 and check valve 23. The pressurized fluid from second port 17 openscheck valve 36 and flows through conduits 32 and 33 of the second supplycircuit into the head end of ram. The check valves 25 and 46 andpressure relief valve 43 prevent pressurized fluid from returning toreservoir. Should an overload above 1,500 psi from the pressure face ofthe pump arise for any reason during this mode of operation relief valve43 will open to dump the fluid to reservoir 41.

For retracting the actuator motor 13 is actuated to reversely drive pump12 in its second mode in which fluid is drawn under suction throughsecond port 17 and directed under pressure through first port 16. Fluidis supplied to the second port from reservoir 18 and filter 21 throughbranch conduit 24 and check valve 25. Pressurized fluid from the firstport is directed through conduit 28 of the first supply circuit to therod end of the ram. Should an overload above 800 psi from the pressureface of the pump arise in this mode, relief valve 45 will open to dumpthe fluid to reservoir through conduit 37 of the return circuit. As theram retracts the pressure from the first supply circuit is communicatedthrough pilot conduit 47 to open pilot operated check valve 46. Thispermits return fluid from the head end of ram to flow through conduit 33of the second supply circuit into conduit 44 of the by-pass circuit andinto reservoir 41 through fine filter 42. The check valve 36 in thesecond supply circuit precludes this return'flow from entering thesecond port of pump 12.

FIG. 2 illustrates an embodiment similar to that of FIG. 1 providing anhydraulic system 50 employing a bi-rotational pump 51 driven by motor 52through drive shaft 53. The construction and operation of pump 51 andmotor 52 are similar to that described for the embodiment of FIG. 1.Circuit means is provided to fluidly interconnect respective first andsecond ports 54, 55 of the pump with the ports 57, 58 of an actuator 59,such as a rotary actuator or extensible hydraulic ram. The circuit meansinclude a bifurcated suction circuit having a conduit 61 drawing fluidfrom reservoir 62 through coarse filter 63 for supply through branchconduits 64, 65 and check valves 67, 68 to the pump ports.

A first supply circuit including conduits 70, 71 connects first port 54of the pump with port 57 of the actuator and a second supply circuitincluding conduits 72, 73 connects the second port of the pump with port58 of the actuator.

A return circuitis' provided and includes a fine filter 74 leading toreservoir 76 from a first branch conduit 77 provided with a firstpressure relief valve 78 and a second branch conduit 79 provided with asecond pressure relief valve 81. Preferably the first and second reliefvalves comprise normally closed valves adapted to open responsive toupstream pressures in the respective supply circuit exceedingpredetermined values.

A by-pass circuit is provided to direct return fluid from port 58 of theactuator to fine filter 74 and reservoir 76 without entering the secondport 55 of the pump. The by-pass circuit includes a conduit 82 and pilotoperated check valve 83 normally precluding flow from the second supplycircuit to the return circuit. A pilot conduit 84 is connected tocommunicate pressure from the first supply circuit for opening checkvalve 83 when the first supply circuit pressure exceeds a predeterminedvalue. A one-way flow valve such as the check valve 86 is provided inconduit 73 to permit flow only in a direction from the pump through thesecond supply circuit. Check valve 86 is connected in the circuit at apoint between the pump and the connection with the conduit leading tocheck valve 83 of the bypass circuit.

The operation of the hydraulic system of the embodiment of FIG. 2 issimilar to that described for the embodiment of H6. 1. Assuming thatactuator 59 is an extensible hydraulic ram having its head end at port58 and its rod end at port 57, it is extended by actuating motor 52 toturn drive shaft 53 in a rotational sense for operating pump 51 in amode which draws fluid under suction through first port 54 anddischarges it under pressure through second port 55 and through thesecond supply circuit into port 58. Return flow from the rod end of theram is directed through conduit 71 where it combines with supply fluidfrom suction circuit conduit 64 for delivery into first port 54 of thepump. For retracting the ram motor 52'is actuated to reversely drive thepump to draw fluid from the reservoir 62 and second port 55 and deliverit under pressure into the first supply circuit and the rod end of theram. Return fluid from the head end of the ram is directed throughconduit 72 where it is blocked from reaching the pump by check valve 86.The pressure in the first supply circuit is simultaneously communicatedthrough pilot conduit 84 to open pilot operated check valve 83 whichdirects the return flow through return circuit 82 and reservoir 76through fine filter 74. This permits the relatively large volume offluid from the head end of the ram to be filtered during its retractionstroke, and in addition the filtering action is positive in that thefluid is forced under pressure through the fine filter.

It will realized in the embodiment of FIG. 2 the second pressure reliefvalve 81 functions to unload pump overpressure condition occurring in aportion of the second supply circuit between pump 51 and check valve 83.This is in comparison to the system in the embodiment of FIG. 1 wherethe pressure relief valve-43 functionsto relieve an overpressurecondition occurring in a portion of the second supply circuit betweenthe ram and check valve 36 under positive pump pressure or back pressurefrom the ram.

FIG. 3 illustrates an embodiment providing an hydraulic system 90employing a bi-rotational pump 91 actuated through drive shaft .92 by asuitable reversible motor 93. Preferably the pump and motor are of thetype described above in connection with the embodiments of FIGS. 1 and2. That is, motor 93 is adapted to be actuated to turn the drive shaftin a rotational sense for operating pump 91 in a first mode which drawsfluid under suction through first port 94 and discharges it underpressure through second port 96, and in a reverse rotational sense tooperate the pump in a second mode drawing fluid under suction throughsecond port 96 and discharging it under pressure through first port 94.

Circuit means is provided for fluidly interconnecting the ports of thepump with a suitable actuator 97. The actuator is illustrated as anextensible hydraulic ram adapted to be mounted, as an example, forraising and lowering the boom of an aerial platform. The circuit meansincludes a fluid reservoir 98 and bifurcated suction circuit having acoarse filter 99 of a suitable size on the orderof one hundred mesh, abranch conduit 101 having a check valve 102 permitting flow only in adirection toward the first port 94 of the pump, and a branch conduit 103having a check valve 104 directing flow only in a direction toward thesecond port 96.

A first supply circuit including conduits 105, 106 directs flow from thefirst port of the pump to port 107 at the rod end of the ram, and asecond supply circuit including conduits 108, 109 directs flow from thesec ond port of the pump through a two-way control valve 111 to port 112at the head end of the ram.

A return circuit is provided and includes first and second branchconduits 113, 114 connected with respective first and second supplycircuits and also connected with a common conduit 116 leading to areservoir 117 through a fine filter 118 of a mesh size on the order of40 microns. Suitable pressure relief valves 119, 120 are provided inrespective first and second branch conduits and preferably comprisenormally closed valves. The valve 119 is adapted to open responsive tothe pressure in first supply circuits exceeding a predetermined value,and the valve 120 is adapted to open responsive to the pressure in thatportion of the second supply circuit between the pump and two-way valve111 exceeding a predetermined value.

Two-way control valve 111 controls the flow to and from the head end ofthe actuator. Preferably control valve 111 comprises a suitable spoolvalve normally urged by the force of pilot assisted, adjustable springservo unit 122 to the illustrated first position which interconnects 109of the second supply circuit with a conduit 123 leading to the conduit114 in return circuit and reservoir 117 through filter 118. The spool ofcontrol valve 111 is urged to its second position by the dischargepressure from pump port 96 communicated through pilot conduit 124 intoservo unit 126. 1n the second position of the spool control valve 111establishes communication between conduit 108 and conduit 109 in thesecond supply circuit to direct discharge fluid from the pump to port112 in the head end of the ram.

Pilot conduits 127, 128 communicate pressure from conduit 106 of thefirst supply circuit into servo unit 122 to act against the end of thespool and assist the spring force in urging the spool to its firstposition for directing return fluid through the filter 118 into thereservoir as the ram is retracting. The spring force urging the spool toits first position may be adjustably varied to selectively vary thepressure limit at which the spool is shifted to its second position forextending the ram.

A pilot operated check valve 129 is provided in a branch conduit 131connected between the suction circuit and conduit 108 of the secondsupply circuit. Check valve 129 normally precludes flow into the suctioncircuit through branch conduit 131, and is adapted to be openedresponsive to the pressure in the first supply circuit exceeding apredetermined value. The pressure in the first supply circuit iscommunicated to check valve 129 by pilot conduit 132 connected withpilot conduit 127. Check valve 129 serves to permit the return fluidfrom the head end of the ram to flow directly to reservoir 98 shouldcontrol valve 111 be inoperable and remain in its second position at thetime pump 92 is actuated to pressurize the first supply circuit. Thispermits the boom to be lowered in an emergency situation where controlvalve 111 fails to function properly.

The operation of the embodiment of FIG. 3 will be described assumingthat actuator 97 comprises an extensible ram mounted for elevating andlowering the boom of an aerial lift. First pressure relief valve 119 isselected to open responsive to a pressure in excess of 800 psi in thefirst supply circuit, and second relief valve 120 is selected to openresponsive to a pressure in excess of 1,500 psi in the second supplycircuit. For elevating the boom motor 93 is actuated to operate pump 91in its first mode drawing fluid under suction through the suctioncircuit and discharging it under pressure into conduit 108 of the secondsupply circuit. The pressure in conduit 108 communicates through pilotconduit 124 to shift the spool of control valve 111 to its secondposition to establish communication from the pump into conduit 109leading to the head end of ram. As the ram extends return fluid from itsrod end is directed through the first supply circuit and into conduitwhere it combines with flow from the suction circuit to supply fluidinto first port 94 of the pump.

For retracting the actuator to lower the boom the motor is reverselyactuated to operate pump 91 in its second mode drawing fluid underpressure through second port 96 and discharging it under pressurethrough first port 94 into the first supply circuit and the rod end ofthe ram. The pressure in the first supply circuit is communicatedthrough pilot conduits 127 and 128 to assist the spring in servo 122 toshift the spool of control valve 111 to its illustrated first position.In this position the return flow from the head end of the ram isdirected into conduit 123 of the return circuit and through fine filter118 into reservoir 117. It will be realized that a relatively largevolume of fluid is filtered in this manner on the retraction stroke ofthe ram. and in addition the fluid is forced through the filter underpressure from the ram.

FIG. 4 illustrates another embodiment providing an hydraulic system 136similar to the embodiment of FIG. 3. This system incorporates abi-rotational pump 137 operated through drive shaft 138 by reversiblemotor 139. The pump and motor are similar to that described inconnection with the foregoing embodiments,

Circuit means is provided for fluidly interconnecting the first andsecond ports 141, 142 of pump 137 with a suitable actuator 143, whichpreferably comprises an extensible ram. The circuit means includes asuction circuit having a reservoir 144 for supply fluid to the pumpports through a coarse filter 146. A check valve 147 in one branch 148of the suction circuit permits flow only in a direction toward the firstport of the pump, and a check valve 149 in the other branch 151 permitsflow only in a direction toward the second port of the pump. A firstsupply circuit having conduits 152, 153 directs flow from the first portof the pump to the rod end of the ram, and a second supply circuithaving a conduit 154 directs flow from the second port through a two-waycontrol valve 156 and conduit 157 to the head end of the ram.

A return circuit is provided to direct return fluid through a conduit158, a fine filter 159 and into a reservoir 160. A first branch 161 ofthe return circuit is provided with a first pressure relief valve 162adapted to open responsive to pressure in the first supply circuitexceeding a predetermined value, and a second branch 163 is providedwith a second pressure relief valve 164 adapted to open responsive tothe pressure in the second supply circuit exceeding anotherpredetermined value.

Two-way control valve 156 preferably comprises a spool valve normallyurged to its first position, as illustrated in FIG. 4, by the force ofpilot assisted adjustable spring servo unit 166. The pressure of secondsupply conduit 154 is communicated to servo 166 through a pilot conduit167. In its first position the spool directs discharge fluid from thepump into conduit 157 leading to the head end of ram. The spool isactuated to its second position by the pressure of first supply conduit153 communicated through pilot conduits 168, 169 into servo unit 171. Inits second position the spool directs return fluid from the head end ofram into return conduit 158 and through the fine filter 159 to reservoir160.

A branch conduit 172 and pilot operated check valve 173 are providedbetween the suction circuit and conduit 154 in the second supplyconduit. Check valve 173 normally precludes flow from the second supplyconduit into the suction circuit and is opened responsive to apredetermined pressure in the first supply conduit communicated throughpilot conduits 168 and 174. With check valve 173 open in this mannerreturn fluid from the head end of the ram is diverted to reservoir 144should control valve 156 be inoperable with its spool valve lodged inthe first position as the ram is retracted.

In operation of hydraulic system 136 it will be assumed that actuator143 is an hydraulic ram mounted to raise and lower the boom of an aeriallift. First relief valve 162 is set to open responsive to the pressurein the first supply circuit exceeding 800 psi, and second relief valve164 is set to open responsive to the pressure in the second supplycircuit communicating with the pump discharge exceeding 1,500 psi. Forextending the actuator is raise the boom motor 139 is actuated tooperate pump 137 in its first mode drawing fluid under suction throughfirst port 141 and discharging it under pressure into the second supplycircuit. With the spool of the control valve 156 urged to its firstposition by the spring force of servo 166 assisted by the pressure inpilot conduit 167, pressurized fluid from the pump is directed intoconduit 157 and the head end of the ram. Return fluid from the rod endis directed through the first supply circuit into conduit 152 where itcombines with flow through branch conduit 148 of the suction circuit tosupply the suction side of the pump.

For retracting the ram to lower the boom. motor 139 is revcrselyactuated to operate pump 137 in its second mode drawing fluid from thesuction circuit and discharging it under pressure into conduit 153 ofthe first supply circuit and the rod end of the ram. The pressure inconduit 153 is communicated through pilot conduits 168 and 169 to shiftthe spool of valve 156 to its second position. Return fluid from thehead end of the ram then flows through conduit 157 and valve 156 intoreturn conduit 158 and through fine filter 159 into the reservoir. Thus,a relatively large volume of fluid is filtered on the retraction strokeof the ram, and moreover the fluid is forced under pressure of the ramthrough the fine filter. FIG. illustrates another embodiment providingan hydraulic system 176 incorporating a pair of birotational first andsecond stage pumps 177, 178 connected to provide duplex, i.e.,two-stage, operation. In one stage flow from the two pumps is combinedfor high volume, low pressure operation, and in another stage one pumpis unloaded for low volume, high pressure operation. The pumpspreferably are of the intermeshing gear pump type as described in theforegoing embodiments, and are mounted for reversible coactuation on adrive shaft 179 powered by reversible motor 181, such as an electricmotor. Motor 181 is adapted to conjointly actuate the two pumps in afirst mode for drawing fluid under suction through their respectivefirst ports 182, 183 and discharging it under pressure through theirrespective seconds ports 184, 185, and in a second mode drawing fluidunder suction through their second ports and discharging it underpressure through their first ports.

Circuit means is provided for fluidly interconnecting the ports of thepumps with a suitable actuator 187, iilustrated as an extensiblehydraulic ram. The circuit means include a suction circuit having aconduit 188 drawing fluid from a reservoir 189 through a coarse tilter191 of a size on the order of one hundred mesh. A

branch 192 of the suction circuit directs flow through a check valve 193and conduits 194, 195 and 196 leading to the first ports of the pumps,and another branch 5 directs flow through a check valve 198 and conduits199, 200 and 401 leading to the second ports of the pumps. Conduit 403provides a first supply circuit directing flow to the rod end of theactuator, and conduit 204 provides a second supply circuit directingflow to the head end of the actuator. A bifurcated return circuitincluding conduit 205 is provided to direct return flow to reservoir 189through a fine filter 201 ofa mesh size on the order of 40 microns. Afirst branch 202 of the return circuit includes a pressure relief valve203 and is connected with the first supply circuit. A second branch 204of the return circuit is provided with a second pressure relief valve206 and is connected through a conduit 207 with conduit 204 of thesecond supply circuit. Preferably the pressure relief valves arenormally closed valves adapted to open responsive to the upstreampressures in the circuits exceeding predetermined values.

Valve means for unloading the first stage pump 177 when the pressure inthe second supply circuit exceeds a predetermined value it is providedand comprises an unloading valve 208 in a branch conduit 209 connectedwith the conduit 401 leading from the second port of the first stagepump, with the outlet of the unloading valve dumping fluid intoreservoir 189. Preferably unloading valve 208 comprises a normallyclosed valve sensing pressure in the second supply circuit throughconduit 207 and a conduit 211, with the valve being set to open whensuch pressure exceeds a predetermined value.

Duplexing valve means is provided to preclude flow from the second stagepump to the reservoir through the open unloading valve during the lowvolume, high pressure stage, and to permit suction flow to the firststage pump during the high volume. low pressure stage. The duplexingvalve means comprises a pilot operated check valve 212 in a conduit 213interconnecting the conduits leading to the second ports of the twopumps.

The check valve normally permits flow from the first stage pump tocombine with that from the second stage pump for high volume, lowpressure supplied to the head end of the actuator. A higher pressure inconduit 213 of the second supply circuit when unloading valve 208 isopen functions to close the duplexing check valve so that a low volume,high pressure from second stage pump 178 is delivered to the head end ofthe actuator. During the low pressure stage duplexing check valve 212opens responsive to a given pressure in the first supply circuit sensedthrough a pilot conduit 214 to permit suction fluid to be supplied topump 177 from check valve 198, conduits 197, 188 and reservoir 189.

A differential volume valve comprising a pilot operated check valve 216is provided in a conduit 217 connected between conduit 204 of the secondsupply circuit and conduit 197 of the suction circuit. The valve 216functions to divert a portion of the greater volume of flow from thehead end of the actuator during retraction into the suction circuit.This valve is opened responsive to the pressure in first supply circuitas sensed through a pilot conduit 218 exceeding a predetermined value.

In operation, it will be assumed that actuator 187 comprises anextensible ram mounted to raise and lower the boom of an aerial lift.The bi-rotational pumps 177, 178 are each sized to produce a 1.5 gpmflow rate with motor 181 operating at 3,450 rpm, thereby providing at2:1 flow ratio between low and high pressure modes. As desired, thepumps could be sized to provide different flow ratios, for example a 3:1ratio between the low and high presssure modes. First pressure reliefvalve 203 is set to open responsive to pressure in the first supplycircuit exceeding 850 psi and second pressure relief valve 206 is set toopen responsive to pressure in the second supply circuit exceeding 1,500psi. The unloading valve 208 is set to open responsive to pressure inthe second supply circuit exceeding 1,000 psi.

For extending the ram to raise the boom, motor 181 is energized toconjointly drive the first and second stage pumps in a mode drawingfluid under suction through their respective first ports 182, 183 anddischarging it under pressure through second ports 184, 185. Fluid fromfirst stage pump 177 is directed through duplexing check valve 212 andthe second supply circuit where it combines with the discharge flow fromsecond stage pump 178 for delivery to the ram through conduit 204. Asthe ram extends and the load on the boom increases, the dual stage pumpdelivery mode is maintained until a pressure of 1,000 psi is exceeded inthe second supply circuit. This pressure is communicated throughconduits 207 and 211 to open unloading valve 208. The discharge fromfirst stage pump 177 then dumps through the unloading valve to thereservoir. This releases the upstream pressure on duplexing check valve212 so that the pressure from second stage pump 178 closes the duplexingvalve preventing second stage flow from discharging to reservoir. Thesystem thus is capable of operating the ram under high pressure, lowflow conditions utilizing relatively lower horsepower requirements ascompared to that which would be required by utilizing the flow from twopumps. As the ram extends return flow from its rod end is directedthrough conduit 403 of the first supply circuit where it combines withthe intake flow from check valve 193 of the suction circuit for deliveryto the first and second stage pumps.

For retracting the actuator to lower the boom, motor 181 is energized toreversely and conjointly drive the two pumps in a low pressure, highvolume mode drawing fluid under suction through their second ports 184,I85 and discharging it under pressure into conduit 203 of the firstsupply circuit and the rod end of the actuator. Pressure in the firstsupply circuit is communicated through pilot conduit 214 to openduplexing check valve 212. This permits intake fluid from check valve198 of the suction circuit to flow through the duplexing check valve andsupply the second port 184 of the first stage pump. Fluid supply to bothpumps in this mode comes from the head end of actuator 187 throughconduits 204, 199, 213, 200 and 401 to pump intake ports 185 and 184.

During the low pressure, high volume mode for retracting the actuator,pressure from the first supply circuit is communicated through pilotconduit 218 to open pilot operated check valve 216. This permits thedifferential volume between the head and rod ends of actuator 187 toflow through this valve and discharge to reservoir 189 through conduits217, 197 and 188 and filter 191.

FIG. 6 illustrates an embodiment of the invention providing an hydraulicsystem 220 incorporating birotational pumps 221 and 222 arranged toprovide duplex, i.e., two-stage, flow to an actuator 223 with provisionfor filtration of return fluid from one end of the actuator. The firststage pump 221 and second stage pump 222 are mounted for conjointreversible operation through a drive shaft 224 from a suitable motor226, such as an electric motor. The two pumps preferably are of thepreviously described gear pump type and are adapted for operationbetween one mode drawing fluid under suction through their respectivefirst ports 227, 228 and discharging it under pressure through theirrespective second ports 229, 230 and a second mode drawing fluid undersuction through their second ports and discharging it under pressurethrough their first ports.

Circuit means is provided for fluidly interconnecting the ports of thepump with actuator 223, shown as an extensible ram. A suction circuitdirects fluid from reservoir 232 through a coarse filter 233, of a sizeon the order of one hundred mesh, through a first branch conduit 234with check valve 236 and a second branch conduit 237 with check valve238. Fluid from the first branch conduit is supplied through conduit 239to the first port of second stage pump 222 and through conduits 241, 242to the first port of first stage pump 221. Fluid from the second branchconduit 237 is supplied to the second port of the second stage pumpthrough conduit 243, and through duplexing check valve 244 into conduit246 to the second port of the first stage pump. A first supply circuitincluding a conduit 247 is provided to direct the flow from the firstports of the pumps to the rod end of the actuator, and a second supplycircuit including a conduit 248 is provided to direct flow from thesecond ports of the pumps through a twoway control valve 249 to the headend of the actuator.

A return circuit including conduits 251 and 252 is provided. to directreturn fluid to the reservoir through a fine filter 253 of a mesh sizeon the order of 40 microns into reservoir 254. A first branch conduit256 having a pressure relief valve 257 and a second branch conduit 258having a second pressure relief valve 259 are provided to relieveoverpressure from the supply circuits. Preferably the pressure reliefvalves comprise normally closed valves adapted to open responsive to theupstream pressure in respective supply circuits exceeding apredetermined value.

Unloading valve means is provided and comprises a normally closed pilotoperated valve 261 in a conduit 262 connected between the first stagepump oulet conduit 246 and the return circuit. A pilot conduit 263 isprovided to communicate second stage pump discharge pressure fromconduits 243 and 264 for opening unloading valve 261 responsive to suchpressure exceeding a predetermined value.

The duplexing check valve 244 comprises a pilot operated check valvewhich normally precludes flow from the second stage pump to reservoir254 when unloading valve 261 is open during the high pressure operatingmode. Pilot conduits 266 and 267 communicate pressure from the firstsupply circuit to open the duplexing check valve permitting fluid to besupplied to second port 229 of the first stage pump when the pumps areoperating in their low pressure mode for retracting the actuator.

Two position flow control valve 249 preferably comprises a spool valvenormally urged by pilot pressure assisted spring servo unit 301 to itsillustated first position for establishing communication between conduit248 connected with the head end of the actuator and return conduit 251,filter 253, and reservoir 254. A pilot conduit'302 communicates pressurefrom the first supply circuit to servo 301 to assist in shifting thespool to its first position when the rod end of the actuator ispressurized. The spool is shifted toits second position by servo unit303 responsive to the pressure communicated through pilot condut 304 ofdischarge fluid from the second port of pump 222. In the second positionof the spool the pump discharge is directed to the head end of theactuator.

A pilot operated check valve 306 is provided in a conduit 307 connectedbetween conduit 243 and the suction circuit. This check valve normalyprecludes flow from the discharge of the second stage pump to thesuction circuit, and is opened responsive to a predetermined pressurecommunicated from the first supply circuit through pilot conduits 266and 308. The function of check valve 306 is to insure that the actuatorcan be retracted should control valve 249 become inoperable with itsspool lodged in its second position. If the control valve does fail tomove from its second position when a retraction stroke is desired, checkvalve 306 will open as the pumps pressurize the first supply circuit,and return fluid from the head end of the actuator is then directedthrough the control valve and diverted through check valve 306 to thesuction circuit.

In operation it will be assumed that actuator 223 comprises an hydraulicram mounted to raise and lower the boom of an aerial lift. Thebi-rotational pumps 221, 222 are each sized to deliver 1.5 gpm flow whendriven at 3,450 rpm by motor 226. First pressure relief valve 257 is setto open responsive to first supply circuit pressure exceeding 850 psi,and second relief valve 259 is set to open responsive to second supplycircuit pressure exceeding 2,000 psi. Unloading valve 261 is set to openresponsive to the second stage pump discharge pressure from conduit 243exceeding 1,200 psi.

For extending the actuator to elevate the boom, motor 226 is energizedto conjointly operate the first and second stage pumps in a mode drawingfluid under suction through the first ports and discharging it underpressure through their second ports. Fluid from first stage pump 221flows through duplexing check valve 244 and combines with flow from thesecond stage pump to communicate pressure through pilot conduit 304 toshift the spool of control valve 249 to its second position. therebydirecting combined flow from the two pumps through conduit 248 to thehead end of the ac tuator. Suction fluid is supplied to the two pumpsfrom reservoir 232, through filter 233, check valve 236 and conduits239, 241, 242. Return fluid from the rod end of the actuator is directedthrough conduit 247 to com bine with the flow into the suction ports ofthe two pumps. This operational mode is maintained until the pressure inthe second supply circuit exceeds 1,200 psi, at which point unloadingvalve 261 is opened due to the pressure communicated through pilotconduit 263. The discharge from first stage pump 221 then flows throughthe unloading valve and filter 253 to reservoir 254. The unloading ofthe discharge from the first stage pump reduces pressure in the freedirection on check valve 244 causing it to close, thereby preventingdischarge .14 from the second stage pump from reaching the reservoir.The second stage pumpthen continues to operate to supply low volume,high'pressure flow to'the head end of the actuator.

For retracting the actuator to lower the boom, motor 226 is reversed todrive'thetwo pumps in a mode discharging fluid under pressurethroughtheir-lfirst ports through conduit 247 tothe rod end of theactuator. Pressure in the first supplycircuit iscommunicated throughpilot conduit 302 to servo unit 301 to assist the spring force inshifting the spool valve to its'first position, thereby directing returnflow fromthe heade'nd of the actuator to return conduit-251', filter253-and reservoir 254. Pressure in the first supply circuit is alsocommunicated'through pilot conduits 266'and '267 to open duplexing checkvalve 244. Thiszpermits suction fluid to flow from conduit264 intosecond port229 of the first stage pump. First circuit pressurealsoopenscheck valve 306 through pilot conduit-266, thereby permitting thedifferential volume fluid which is not supplied to the suction ports ofthe pumps to be discharged through conduit 307 to reservoir 232 throughfilter 233 in the event that valve 249 sticks in its second position.

While the foregoing embodiments are at present considered to bepreferred it will be understood that variations and modifications may bemade therein by those skilled in the art and it is intended to cover allsuch variations and modifications as fall withinthe true spirit andscope of the invention as expressed in the appended claims.

We claim:

1. ln a system for operating an hydraulic actuator, the combination of afirst stage reversible pump, a second stage reversible pump, each pumphaving first and second ports and being capable of operating in a firstmode in which its first and second ports assumerespective suction andpressure functionsand in a second mode in which its ports assumerespective pressure and suction functions, fluid circuit means fordirecting fluid between the ports of the pumps and said actuator, saidcircuit means including fluid reservoir means, a suction circuit todirect fluid from the reservoir means to said ports, a first supplycircuit to direct fluid under pressure from the first ports of saidpumps to the actuator, a second supply circuit to direct fluid underpressure from the second ports of said pumps to said actuator, unloadingvalve means adapted to open for directing fluid from the second port ofthe first pump to the reservoir means responsive to pressure in theportion of the second supply circuit between the second port of thesecond pump and the actuator exceeding a given value, and duplexingvalve'means for precluding fluid flow from the second port of the secondpump to the reservoir means when the loading valve means is open.

2. A system as in claim 1 in which the circuit means includes a returncircuit to direct fluid from said actuator to the second ports and tothe reservoir means when the pumps are operated in their second modes.

3. A system as in claim 1 in which said return circuit includes valvemeans operable to open for directing return fluid from said portionofthe second supply circuit to the suction circuit responsive to thepressure in the first supply circuit exceeding an established value.

4. A system as in claim 1 in which said duplexing valve means includespilot operated check valve means operable to direct flow from thesuction circuit to the second port of the first pump responsive to thepressure in said first supply circuit exceeding another given value whensaid pumps are operated in their second modes.

5. A system as in claim 1 in which the circuit means includes a returncircuit to direct return fluid from the actuator to the reservoir means,filter means for filtering fluid in the return circuit, and flow controlvalve means operable for directing flow from the first and second stagepumps to the actuator through the second supply circuit when thepressure in the latter exceeds a predetermined value, said control valvemeans being further operable for directing return flow from the actuatorthrough the return circuit and filter means to the reservoir meansresponsive to the pressure in the first supply circuit exceeding anotherpredetermined value.

6. A system as in claim 5 in which the duplexing check valve meansincludes pilot operated check valve means operable to direct flow fromthe suction circuit to the second port of the first pump responsive tothe pressure in said first supply circuit exceeding another given valuewhen said pumps are operated in their second modes.

7. A system as in claim 5 which includes pilot operated check valvemeans for permitting flow from the portion of the second supply circuitbetween the second port of the second pump and the control valve meansto the reservoir means responsive to pressure in the first supplycircuit exceeding another given value to permit release of return flowfrom said actuator through the control valve means should the latter berendered inoperable.

8. A system as in claim 5 which includes first pressure relief valvemeans to direct fluid from the actuator to the reservoir means throughthe first supply circuit when the pressure therein exceeds a firstpredetermined value, and second pressure relief valve means fordirecting fluid from the actuator to the reservoir through the secondsupply circuit when the pressure therein exceeds a second predeterminedvalue greater than said first value.

9. A system as in claim 1 which includes means for co-actuating saidpumps in a common selected mode for pumping fluid under pressure intoselected first or

1. In a system for operating an hydraulic actuator, the combination of afirst stage reversible pump, a second stage reversible pump, each pumphaving first and second ports and being capable of operating in a firstmode in which its first and second ports assume respective suction andpressure functions and in a second mode in which its ports assumerespective pressure and suction functions, fluid circuit means fordirecting fluid between the ports of the pumps and said actuator, saidcircuit means including fluid reservoir means, a suction circuit todirect fluid from the reservoir means to said ports, a first supplycircuit to direct fluid under pressure from the first ports of saidpumps to the actuator, a second supply circuit to direct fluid underpressure from the second ports of said pumps to said actuator, unloadingvalve means adapted to open for directing fluid from the second port ofthe first pump to the reservoir means responsive to pressure in theportion of the second supply circuit between the second port of thesecond pump and the actuator exceeding a given value, and duplexingvalve means for precluding fluid flow from the second port of the secondpump to the reservoir means when the loading valve means is open.
 2. Asystem as in claim 1 in which the circuit means includes a returncircuit to direct fluid from said actuator to the second ports and tothe reservoir means when the pumps are operated in their second modes.3. A system as in claim 1 in which said return circuit includes valvemeans operable to open for directing return fluid from said portion ofthe second supply circuit to the suction circuit responsive to thepressure in the first supply circuit exceeding an established value. 4.A system as in claim 1 in which said duplexing valve means includespilot operated check valve means operable to direct flow from thesuction circuit to the second port of the first pump responsive to thepressure in said first supply circuit exceeding another given value whensaid pumps are operated in their second modes.
 5. A system as in claim 1in which the circuit means includes a return circuit to direct returnfluid from the actuator to the reservoir means, filter means forfiltering fluid in the return circuit, and flow control valve meansoperable for directing flow from the first and second stage pumps to theactuator through the second supply circuit when the pressure in thelatter exceeds a predetermined value, said control valve means beingfurther operable for directing return flow from the actuator through thereturn circuit and filter means to the reservoir means responsive to thepressure in the first supply circuit exceeding another predeterminedvalue.
 6. A system as in claim 5 in which the duplexing check valvemeans includes pilot operated check valve means operable to direct flowfrom the suction circuit to the second port of the first pump responsiveto the pressure in said first supply circuit exceeding another givenvalue when said pumps are operated in their second modes.
 7. A system asin claim 5 which includes pilot operated check valve means forpermitting flow from the portion of the second supply circuit betweenthe second port of the second pump and the control valve means to thereservoir means responsive to pressure in the first supply circuitexceeding another given value to permit release of return flow from saidactuator through the control valve means should the latter be renderedinoperable.
 8. A system as in claim 5 which includes first pressurerelief valve means to direct fluid from the actuator to the reservoirmeans through the first supply circuit when the pressure therein exceedsa first predetermined value, and second pressure relief valve means fordirecting fluid from the actuator to tHe reservoir through the secondsupply circuit when the pressure therein exceeds a second predeterminedvalue greater than said first value.
 9. A system as in claim 1 whichincludes means for co-actuating said pumps in a common selected mode forpumping fluid under pressure into selected first or second supplycircuits.